Just access the GPS location, three buttons (Start polygon, Add point and Close Polygon). Flow: Press start polygon automatically adds first point, move around, press Add point, and adds next point and draw a line between points. When I press Close polygon, it will draw a line to first point created. All points of polygon should be stored somewhere and each point should contain the GPS coordinates.
Note: The app should work offline and I will not pay more the 10$ for this simple app. So if you want more search for other projects or come with some extra implementations.